The present invention relates to apparatus for removing volatile substances or the like from viscous material, such as a polymer substance, by a process termed devolatilization. The polymer material which is to be devolatilized is fed into a cylindrical cage and carried through the cage by screw members. The polymer material is moved through a vented section of the cage, where the pressure on the polymer is substantially reduced, permitting the volatile substance to vaporize and escape. The devolatilized polymer is then carried into an enclosed screw section for further working or for extrusion sizing.
One such devolatilizing device is shown in U.S. Pat. No. 3,684,252, issued Aug. 15, 1972, to Nissle et al and assigned to the assignee of the present invention. Nissle et al discloses a devolatilizing apparatus in which the vent section includes a number of slit-like vent openings around the periphery of the devolatilizer cage. The vented vapors from the devolatilizer cage pass through the vent openings and the volatile substance may thereafter be recovered. Such vent openings may, however, become clogged during extended use. As the volatile substance evaporates in a vent section, small pieces of polymer, termed "fines," may be carried by the vapor and deposited in the vent or in other portions of the vapor recovery system.
The problem of keeping a vent in such a device free of polymer material has been recognized in the past. Several vent arrangements have incorporated static structures designed to prevent clogging. U.S. Pat. No. 3,350,742, issued Nov. 7, 1967, to Wood, shows a vent including a vent liner which is to be replaced periodically. U.S. Pat. No. 3,737,150, issued June 5, 1973, to Otake, discloses a vent arrangement in which the vent tube extends through the wall of the cylindrical cage, with the inner end of the tube terminating adjacent to the peripheral surface of the screw shaft. A notch in the end of the vent tube is intended to prevent the material being worked from moving up the vent tube. U.S. Pat. No. 3,781,132, issued Dec. 25, 1973, to Latinen, discloses a devolatilizer having a vent which includes wedge shaped members at the vent opening which prevent the polymer from moving into the vent.
A different approach is taken in U.S. Pat. No. 3,212,133, issued Oct. 19, 1965, to Heidrich. The Heidrich patent discloses an extruder having a vent duct which includes a screw conveyor for returning material to the cylindrical cage of the extruder, should such material be forced into the vent duct. The screw conveyor is supported in the vent duct by a bearing arrangement which permits it to be displaced axially. A limit switch detects when the screw conveyor is moved axially away from the extruder by material entering the vent duct from the cylindrical cage. The limit switch actuates a drive motor which rotates the screw conveyor, thereby returning the material to the cylindrical cage of the extruder.
While the Heidrich vent arrangement provides a positive mechanism for cleaning the vent in an extruder, actuation is dependent upon the displacement of the screw conveyor away from the extruder and such displacement occurs only when a substantial bulk of extruder material is displaced upwardly into the vent. Heidrich provides no solution to the problem of fines being entrained in escaping gases since such fines will not be of a bulk sufficient to displace the screw conveyor and cause it to be rotated. Additionally, the helical channel defined by the vent and the screw conveyor flights is relatively small in cross-sectional area. A high vapor velocity of the escaping vapor will result and accentuate the problem of fine entrainment.
In U.S. Pat. No. 3,963,558, issued June 15, 1976, to Skidmore, and in U.S. Pat. No. 3,799,234, issued Mar. 26, 1974, to Skidmore, are disclosed screw vent arrangements in which the vents each have positioned therein two interlocking screws which continuously clean the vent. The cross-sectional area for the escape of the vented gases is purposefully made small in order to act as a mechanical filter. Such an arrangement, however, results in an extremely high velocity for the vapor escaping from the devolatilizing apparatus, with the result that significant fines may be entrained in the gas.
It is seen, therefore, that a need exists for a self-cleaning venting arrangement for a devolatilizer in which the problem of fine entrainment is minimized.